There is known a traditional cooling system of an internal combustion engine, in which an oil temperature sensor is disposed in an oil supply passages adapted to supply oil to an oil jacket formed in a cylinder head, at a position on a rear surface of a cylinder block and above a crankcase (see e.g. Japanese Patent Laid-open No. 2006-97614).
Incidentally, in the cooling system of the internal combustion engine described in Japanese Patent Laid-open No. 2006-97614 mentioned above, the oil temperature sensor is disposed perpendicularly to a cylinder-arrangement direction so that it is arranged to project from the cylinder block. This limits the flexibility of arrangement of peripheral machinery included in the internal combustion engine.